1. Field of the Invention
The present invention relates to a coaxial loudspeaker and more particularly to a coaxial loudspeaker which incorporates an acoustic low pass filter therein to eliminate distortion.
2. Description of the Prior Art
U.S. Pat. No. 2,822,884, entitled Loudspeaker Enclosure, issued to Edgar H. Simpson on Feb. 11, 1958, teaches a single speaker cabinet with two acoustic filters and a single speaker. U.S. Pat. No. 2,866,514, entitled, Corrective Loud Speaker Enclosure, issued to Paul Weathers, on Dec. 30, 1958, teaches a single speaker enclosure with a plurality of chambers which are acoustically coupled to the speaker chamber by acoustic filters.
U.S. Pat. No. 2,067,582, entitled Sound Filter for Loudspeakers, issued to Edward Sperling on Jan. 12, 1937, teaches a sound filter used with only one loudspeaker. The sound filter, when it is applied to the loudspeaker, functions to filter and to clarify the sounds and tones emitted therefrom by minimizing harshness, distortion, static or interference while serving to generally improve the quality of the sounds or tones.
U.S. Pat. No. 2,656,004, entitled Multisection Acoustic Filter, issued to Harry F. Olson on Oct. 20, 1953, teaches a multisection acoustic filter which consists of one or more stages or sections. Each section includes a pair of parallel, perforated sheets or plates separated from each other a suitable distance and joined at their peripheries in any appropriate manner to enclose an air space therebetween. Two such plates constitute a single section filter. A two section filter consists of three such plates, one being common to each section; a three section filter consists of four such plates. These filters may be placed in front of any sound source, such as the loudspeaker of a radio receiver, for example, or in proximity to one or more musical instruments or the like to reduce the high frequency response in each case.
A two-way loudspeaker system is a very practical solution to the problem of building a transducer array that will cover the full audio frequency range. The coaxial arrangement, where the low frequencies are reproduced by a cone loudspeaker of a diameter in the range of twelve to fifteen inches (called a woofer) and the high frequencies are reproduced by a small cone or horn transducer (called a tweeter) mounted in front of the larger cone, provides advantages over the spaced woofer-tweeter arrangement in regards to producing an even distribution of sound at angles other than directly on axis. This is due to the closer spacing of the radiating elements. A further advantage in the smoothness of frequency response can be obtained if the tweeter horn is disposed so that it projects through the center pole piece of the low frequency transducer, with the horn continuing forward approximately to the plane of the rim of the woofer. In this configuration the acoustic centers of the two transducers can be arranged to superimpose each other at their crossover frequency by adding a small amount of electrical time delay in the woofer electrical crossover network. The superimposition of the acoustic centers of the two transducers is verified by acoustical phase measurements. The coaxial configuration however, as typically found in commercial loudspeakers has a problem with intermodulation distortion. The audible distortion of the high frequencies radiated by the tweeter is caused by the Doppler shift as these high frequencies are reflected off the moving cone surface of the low frequency woofer.
Paul W. Klipsch, in an article entitled "A Note on Modulation Distortion: Coaxial and Spaced Tweeter-Woofer Loudspeaker System", published in the Journal of the Audio Engineering Society, Volume 24, Number 3, April, 1976 on pages 186 and 187, discusses the FM distortion of two loudspeaker systems, one of which has a tweeter mounted coaxially with the woofer, and the other has a spaced tweeter-woofer configuration. A loudspeaker radiating high frequencies in close proximity to a loudspeaker radiating low frequencies is observed to be subject to modulation distortion. Thus a tweeter being fed f.sub.2 =9559 Hz in proximity to a bass speaker radiating f.sub.1 =50 Hz was found to radiate side frequencies of 9609 , 9509, 9659 (f.sub.2.sup..+-. f.sub.1, f.sub.2.sup..+-. 2f.sub.1, . . . ). The sound from the tweeter diffracts around the horn and is reflected by the moving woofer cone, thus producing FM distortion. Klipsch found that clearly audible FM (frequency modulation) distortion of the f.sub.2 component of 9559 Hertz was produced by a 50 Hertz, f.sub.1, signal of 95 db, sound pressure level in the coaxial arrangement. The total root mean square modulation distortion was 27 decibels below the level of f.sub.2. The magnitude of the distortion components which are generated in this manner is determined by the following equation: EQU d=0.033A.sub.1 f.sub.2 k,
where d=total root mean square value of the distortion sidebands as a percent of the amplitude of the higher modulated frequency, f.sub.2, and A.sub.1 =peak amplitude of motion in inches at the lower modulating frequency, f.sub.1, and k=the proportion of high frequency sound which is radiated to the rear of the tweeter and reflected off the moving low frequency cone.
For example, if A.sub.1 =0.25 inches, f=5000 Hertz, k=0.1, which is minus twenty decibels, the distortion, d, is 4.1 percent, which is -27.7 db. This degree of distortion would be clearly audible.
A. Stott and P. E. Axon, in their article entitled, "The Subjective Discrimination of Pitch and Amplitude Fluctuations in Recording Systems", published in the Journal of the Audio Engineering Society, Volume Five, Number 3, July, 1957 beginning on page 142, discusses the threshold of audibility of frequency modulation distortion of recorded piano program material. Referring to their FIG. 10, it can be verified that 0.4% RMS FM distortion by 30 Hz is the audible FM distortion threshold, of this musical material.
In a conventional coaxial speaker a portion of the high frequency sound from the horn is radiated toward the cone, which is moving and which reflects the high frequency sound, thereby creating a Doppler intermodulation-distortion. An acoustic low pass filter, if it is placed between the horn and the cone, will attenuate the high frequency sound traveling from the horn to the cone and from the cone to the environment thereby dramatically reducing the Doppler intermodulation-distortion.
As an example, if an acoustic filter of the full section type, which has a cutoff frequency of 2500 Hertz, is fitted between the tweeter and woofer, at 5000 Hertz, the factor k in the example cited above would be reduced by approximately forty decibels (40 db) to 0.001, and the distortion would also be reduced by forty decibels, to 0.041 percent. This degree of distortion would be approximately 20 db below audibility. A full section filter attenuates as much as twenty decibels at one octave above the cutoff frequency and the k factor includes two passes through the filter thereby providing the forty decibel reduction as calculated.
This distortion reduction afforded by such a filter increases as the frequency f.sub.2 increases. Without an acoustic filter the distortion increases in a manner directly proportional to the frequency radiated by the tweeter.
Furthermore, the low pass filter attenuates the harmonic distortion components which are emanating from the cone at frequencies above the cutoff frequency of the acoustic filter which in a typical application is designed to be at the same frequency as the electrical cross-over between the woofer and the tweeter loudspeakers.